1. Technical Field
This invention involves a mechanism for latching and unlatching a moving arm, such as the head arm of a disk drive, or other moving member, particularly when power is shut off or fails.
2. Description of Related Art
During the normal operation of a hard disk drive, the read-write heads are aerodynamically lifted above the disk media surface several millionths of an inch by the relative air velocity associated with the spinning disks. When power fails or is intentionally shut off, it is important to move the heads to a safe or "parked" position that is not over the data stored on the disks: if the heads were to come into physical contact with the data media surface, there is a significant probability that the data in the region of contact could be disturbed or lost.
Furthermore, it is important that the heads be kept in a safe parked position while the disk drive is not in use. Portable, laptop and other small computers that contain hard disk drives are particularly susceptible to such head-media contact problems since they are carried about and may be used in mobile environments in which they are jostled, bumped or subjected to other forces. During such motion, the heads could be caused to move radially, which could affect the media material if in contact with the data surface. The head-arm assembly itself, moreover, may also be damaged by too rough handling if it is allowed to move freely. In order to avoid such damage to the data or to the head-arm assembly, the head-arm assembly is locked in a safe position.
Historically, larger computers, which were wired to special power supplies, could be switched to an emergency back-up power source when a power failure occurred. While on back-up power, a special shut-down program saved data being processed, moved the head-arm assembly to a safe parked location, and then shut down the computer system. Most modern computer users, however, do not invest in such "un-interruptable power supplies". In the case of portable computers, users would prefer not to carry such relatively heavy, special power supplies.
In the interests of saving weight and power, particularly for desk-top and portable computers, a scheme has evolved that utilizes the momentum, or, more properly, the kinetic energy stored in the spinning data disks to generate sufficient power to carry out this head-arm parking strategy. According to this known scheme, when a voltage-sensing circuit senses power failure, the circuit switches the drive circuit that controls the disk's spindle motor to immediately cause the spindle motor to act as a generator. The voltage thus generated as the disk spins down is used to drive the head-arm assembly toward the safe position, and can also be used to arm a latch mechanism, which secures the head-arm assembly in the safe position (when it arrives in that position).
The way in which the switching is arranged is typically that a spindle motor driver circuit sends current to the spindle motor via one or more transistor switches, each of which has a first state (motor mode) and a second state (generator mode). The transistors are held in the first state by the voltage from the disk drive power supply; the motor and the attached media disks are thereby caused to accelerate to operating speed and are maintained at that speed by the spindle motor driver circuit, which contains a means of detecting the spindle motor speed.
When the power supply voltage fails or is purposely turned off, the transistor switches assume their "normal" state--the second state--which connects the spindle motor leads to the parking and latch circuits. The switch from the first state to the second state is thus caused by the absence of voltage (similar to a "normally closed" relay contact closure when the relay coil is turned off).
The parking and latch circuit is typically composed of simple transistor switches that are operated by the voltage now present at the spindle motor leads by virtue of the rapid rotation of the spindle motor and the disks, which together have a high moment of inertia and thus, at a high rotational speed, a significant amount of kinetic energy that can be converted at least partially into usable electric current. This parking and latch circuit sends current to the head-arm actuator to turn the actuator toward the direction of the latch.
A commonly used latch consists of a small magnet that attracts a ferro-magnetic piece of metal mounted on the head-arm assembly, usually near the outboard end of the actuator coil mount. This method has the advantage of low cost and simple assembly operations. One of the disadvantages of this method, however, is that it suffers from low latch force--the head-arm assembly may be easily jarred loose from this latch during transport.
Another disadvantage of this conventional method is that the magnetic field from the latch may affect the motion of the head-arm assembly itself (even when the assembly is not to be latched) when the assembly is near the latch. Special circuitry must often be added to the actuator servo system to compensate for this magnetically caused force variation.
A third disadvantage is that the force that the head-arm actuator can provide to unlatch itself during normal start-up and running is inherently limited, and this in turn limits the latching force that the latch may apply to the assembly. The stronger the attraction between the ferro-magnetic piece and the permanent magnet, the greater is the force required for the head-arm assembly to "free" itself during start-up. Some systems have even become "stuck" that is have been unable to unlatch the head-arm; latched computers with low battery voltage (for example after long use without recharging) have proven particularly vulnerable to this problem.
Other known latch mechanisms are purely mechanical. These mechanisms include springs, ramps, spring-loaded clips and other devices that rely on the arm "jamming itself" and are based on the velocity of the head-arm assembly as it is accelerated toward the "safe position". These devices, however, have proven unreliable and difficult to adjust during assembly. If the clip is too tight or narrow, for example, the arm will either not be able to jam itself securely into the safe position, or it will require too much force to free it once it has. Conversely, if the clip is too "loose" or open, it will not hold the assembly securely enough and the assembly may work itself free.
Another conventional latch concept uses an intermediate linkage or lever to locate the magnetic latch away from the vicinity of the actuator coil or media disk. Associated with this linkage is a solenoid mechanism that may include an internal permanent magnet that locks the linkage in place when the power is off. This concept, which is generally more reliable than other conventional methods, is relatively expensive and may have to be adjusted to have the proper holding forces during the production assembly operation. Such linkage systems, especially those that include solenoid actuators, also have relatively many moving parts, each of which increases the complexity, cost and risk of failure of such systems. Furthermore, this type of latch assembly occupies more precious space in the disk drive than the simple magnet latch described above.
The disk drive industry therefore lacks and needs a head-arm latch system that meets the following requirements:
1. The head-arm should stay locked in the safe position when no power is applied to the system, regardless of external forces that tend to jar the head-arm assembly from this position. No power should be consumed to keep the assembly in this locked position.
2. Upon power-up, the head-arm assembly must be easily freed from the latch. The power consumed during this unlatching operation may, however, be relatively high for a short period.
3. During normal disk drive operation, the latch should remain stored in an unarmed state without power consumption. Relatively little force should be required to hold the latch in this state. After the head-arm is released from the latch, the influence of the latch on the performance of the head-arm assembly when the assembly is near the latch should be minimized. There should be no interference with the disk drive operation while in this mode; however, it may be permissible for the latch to become armed and actually capture the head-arm assembly under conditions of extreme shock. While such capture is inconvenient, it should be able to be corrected quickly through a retry of the seek operation.
4. Upon power failure, the latch must arm itself and mechanically immobilize the head-arm assembly when the assembly arrives at the safe position. Power consumption should be relatively low and of short duration (preferably only a few milliseconds) during this operation.
Although not directly related to the efficiency of the latch mechanism itself, it would also be advantageous for the bearings used to mount the moving parts in the latch mechanism to have a long life, while still allowing for rapid acceleration, yet be small enough not to hinder the high degree of miniaturization now sought after in modern disk drives. Furthermore, the amount of particles created as the moving parts of the bearing wear should be kept to a minimum, and preferably no particles should be created at all; this reduces contamination and ensures smooth latch motion.
The object of this invention is to provide a latch mechanism that meets these requirements. Although the invention is especially well-suited for use in latching head-arm assemblies in disk drives, it is also advantageous when used to latch in place other sensitive moving elements that are found in many instruments and other devices.